The production of semiconductor devices such as integrated circuits or microwave transistors, in general, requires that diffused semiconductor regions of one conductivity type be formed in a body of another conductivity type, thereby producing a junction line on the surface of the body. Since contacts must subsequently be made to the diffused region, reduction in size of such devices is limited by the fact that the contact cannot touch both sides of the junction or it will short out the junction.
It has been well known to mask a semiconductor body with a layer of dielectric material and to diffuse the regions through apertures of the mask. Due to lateral diffusion, the junction line is formed underneath the mask and the distance from the mask opening is a function of the time allowed for the diffusion process which also controls the distance into the body which the junction moves. It has heretofore been necessary to diffuse such junctions sufficiently deeply into the body, for example greater than one micron deep, to produce a junction on the surface of the body over one micron from the edge of the mask. Otherwise, the layer of oxide and/or other material which is formed on the surface of the semiconductor in the wash apertures during such emitter diffusion and which was removed by a chemical etch which proceeds in all directions through the oxide would undercut the oxide mask enough to uncover and thus spoil the junction. This additional lateral diffusion requirement in turn results in a production requirement that additional space on the semiconductor chip must be reserved to prevent the junction from interfering with the next adjacent semiconductor element or isolation region and, hence, restricts the number of junctions which can, as a practical matter, be formed on a given area of semiconductor surface. In addition, the additional depth of the junction reduces control of the thickness of the base region and, hence, for production purposes, limits the upper frequency response of a circuit on a chip containing a substantial number of interconnected semiconductor elements.
Alternatively, multiple masking steps to produce an emitter junction area of one size and a smaller mask aperture for subsequent formation of emitter contacts requires very precise mask locationing of the subsequent masks.